The present invention generally relates to methods and apparatus for the application and dosage of pressurized coating material onto a moving base. The moving base may comprise paper or board web in which case the invention relates to an improvement in coating techniques which are generally known in the paper industry.
The moving base may also comprise, for example, the surface of a roll of a size press. The coating applied to the roll surface is transferred onto the paper web passing through the size press in the gap between the press rolls. In this case, the invention relates to an improvement in surface-sizing techniques generally known in the paper industry.
More specifically, the present invention relates to a method for the application and dosage of a coating material onto a moving base, such as a paper or board web, the surface of a roll or counter-roll or the like wherein the coating material is fed under pressure into a coating material chamber of a coating device defined by a region of the moving base to be coated, a coating member extending substantially transversely with respect to the direction of movement of the base at one edge of the moving base region and a front member extending substantially transversely with respect to the direction of movement of the base at a front edge of the base region.
Additionally, the invention also relates more specifically to apparatus for carrying out the method of the invention including a coating device defining a coating material chamber of the type described above.
Conventional short-dwell units used, for example, in the coating of paper webs include arrangements in which coating material is fed under pressure into a coating chamber defined by a coating blade on the one hand, and by a front wall, e.g., a dam blade, on the other hand. A gap is formed between the front wall and the web to be coated. By feeding coating material into the coating chamber at a sufficiently high volumetric flow rate and pressure, an overflow or reverse flow, i.e. a flow in the direction opposite to the running direction of the web, takes place through the gap between the front wall of the web which functions to seal the gap so that air cannot penetrate into the coating chamber.
Conventional coating arrangements of this type are not entirely satisfactory. A coating device is disclosed in Finnish Pat. No. 61,534 in which the coating material is applied onto the base to be coated under pressure at a certain volumetric flow rate. When this arrangement is used in coating paper, it has been difficult to prevent the passage of air through the gap formed between the paper web and the front wall situated forwardly of the doctor blade or coating member of the coating device in the running direction of the web. The entry of air into the coating chamber has resulted in unevenness in the transverse profile of the coating applied to the web.
Although it is possible to prevent the entry of air into the coating chamber by tightly pressing the front member against the web, this has the effect of causing fibers and other impurities to be ground from the web into the coating chamber where they adhere to the coating blade resulting in blade streaks occuring in the coating. When the base being coated comprises a roll or rolls of a size press, excessive pressure between the front wall of the coating material chamber and the surface of the roll may cause wear damage in the roll surface since the impurities in the paper web tend to remain in front of the front wall due to the excessive pressure.
Another conventional arrangement for coating a moving base, and, in particular, a moving web is disclosed in U.S. Pat. No. 4,250,211. In this arrangement, the sealing of the front wall of the short-dwell unit with respect to the moving web to prevent passage of air through the gap formed between them is achieved by causing the coating material to flow through the gap in a direction opposite to the running direction of the web to completely fill the gap. Since the magnitude of the gap between the web and the front wall is typically in the range of between about 3 to 6 mm, a flow of coating material must typically be in the range of between about 20 to 30 times as large as the quantity of coating material that remains in the web in order to completely seal the gap. When the surface of a roll is being coated, the magnitude of the flow of coating material necessary to seal the gap, although being smaller than that required to seal the gap when coating web material, is still quite large as compared to the coating material layer that remains on the surface of the roll. When the gap is large, the pressure in the coating material chamber cannot be easily increased to a very high level, since the flow resistance through a large gap is quite low. Thus, if the pressure in the coating material chamber increases, the overflow or reverse flow through the gap correspondingly increases. If the gap size is reduced, for example to 1.5 mm, even small variations in the gap width caused, for example, by manufacturing defects, will manifest themselves as defects in the transverse profile of the coating layer. On the hand, in the case of gaps of reduced width, the pressure in the coating material chamber may become so high due to high flow resistance presented by the reduced gap, that problems in the sealing of the edges of the coating material chamber may result.
Attempts have been made to avoid the drawbacks of the arrangements of the type disclosed in U.S. Pat. No. 4,250,211 as discussed above through the use of coating devices in which the coating material is pressed within the region or zone of application of the web surface by means of hydrodynamic forces. In this connection, reference is made to the arrangements disclosed in U.S. Pat. No. 4,369,731 and German publication No. 3,338,095. It is, however, noted that in such arrangements, the hydrodynamic forces are utilized in the application area or zone of application of the coating material itself, so that it has still been necessary to pass a large overflow or reverse flow of coating material from the gap between the front wall of the application zone and the web to be coated in the inlet direction of the web.